Catenary Line Dynamic Motion Suppression

ABSTRACT

Dynamic motion decoupling is effected with the use of mass, added mass, buoyancy, submerged weight and drag in areas of relatively low tension. High curvatures of lines on some configurations, together with their low slope may be utilized. The original line configuration may or may not be modified. Known motion suppressing device designs can be used. Because of the low slope on some configurations, said motion suppressing devices can be installed on arbitrarily long line segments to achieve objections required. Novel, drag and added mass enhancing devices effective in all directions can be used to increase the suppression effectiveness and/or in order to reduce the number of devices used. This invention is suitable for use on new designs and it is also suitable for retrofitting on existing, already installed lines.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional PatentApplication Ser. No. 60/593,269 filed Jan. 3, 2005 and entitled:“Catenary Line Dynamic Motion Suppression Arrangement” the disclosure ofwhich is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to lines used to connect undersea equipment torelated equipment on or near the surface.

2. Description of the Related Art

Petroleum exploration and production is increasingly being conductedoff-shore and at ever deeper locations. Typically, a mobile offshoredrilling unit (“drilling rig”) is used to create a well. Once the wellis completed, a production platform or a buoy is installed at the siteto recover the petroleum products which may subsequently be loaded ontoa tanker or pumped via pipelines to on-shore facilities.

Exploration and production platforms take many forms. The appearance andbasic features of various types of offshore platforms appearance andbasic features of various types of offshore platforms are obvious toanybody skilled in offshore engineering and are widely described intechnical literature. Examples include ships (mostly tanker-likeFloating Production Systems—FPSs and FPSOs—FPSs with off-loading),semi-submersibles (including deep draft semisubmersibles), Tension LegPlatforms (TLPs), compliant and articulated columns and towers, guyedtowers, SPAR platforms, jacket (fixed) platforms and jack-up rigs.

It is noted, that many riser, umbilical, hose, cable, etc. lines thatare relevant to this specification have their top ends supported forexample by buoys, columns, etc. that cannot be classified as platforms.

Lines that are relevant to this specification are used in order to:

-   -   transport fluids in both directions between locations at or near        the surface and at or near the bottom (examples include import        and export lines transporting hydrocarbons, water and gas        injection lines, gas lift lines, etc.),    -   transfer electrical and hydraulic power,    -   transfer information, including control, monitoring, data,        telecommunication,    -   transfer loads (examples: tendons, tethers, cold tubing, etc.,        many risers deployed share mooring loads with ‘regular’        moorings).

Lines feature a variety of prior art configurations that are used inoffshore and onshore engineering. The two major classes of linesinclude:

-   -   catenary lines (examples: flexible risers, Steel Catenary        Risers—SCRs, umbilicals, hoses, jumpers, cables),    -   tensioned lines (examples: tensioned risers including        freestanding and hybrid risers, and tendons or tethers).        Most of the said lines are relevant to this specification and        they are referred to herein as ‘lines’. Many line configurations        are used in marine engineering, their basic features are        well-known to those skilled in the art, and they are well        described in technical literature.

For example Barltrop¹ depicts and describes a representative (but notcomplete) selection of prior art line configurations used in offshoreengineering. Many of the line configurations known are referred toelsewhere in this specification.

U.S. Pat. No. 5,222,453 demonstrates a use of mass enhancing devicesmounted on mooring lines and utilized to modify dynamic motions of amoored structure, without affecting static loads in the mooring system,where axial line dynamics is of primary importance. These were of littlerelevance to this invention that is related to different kinds of lines,and primarily, but not exclusively, to transverse linedynamics—transverse motions and bending of risers, umbilicals and hoses.

For the purpose of this specification, in most cases, the details ofline description (example: flexible riser, hose or umbilical or even anSCR) is of secondary importance or even of no importance. This isbecause different lines are subject to the same physics, the same harshenvironment and there are many similarities between equipment used withvarious line configurations, with lines constructed in differing ways,(including using different materials) as well as lines used for vastlydiffering functional purposes.

A general description and explanation follows of technical issues inoffshore and onshore engineering, including problems, as relevant tothis invention, as well as that of prior art in the mitigation of someof the said problems.

In particular a simple (free-hanging) catenary configuration, as well asin many implementations of other line configurations are known toexperience significant movement near the seabed and interactions withthe seabed and/or with structures at the seabed ends of the lines. Theextent of these movements, together with the variations in the valuesand the sign of the effective tension and the variations in the radii ofcurvature of the said lines, in particular but not exclusively near theseabed, are mitigated by this invention.

Risers and mooring lines are used in many design configurations thatinclude various applications of negatively buoyant clump weights anddistributed weights, approximately neutrally buoyant lines and devicesas well as positively buoyant discrete and distributed, positivelybuoyant elements and segments. By stating that a line is neutrallybuoyant it is meant herein that the line is either neutrally buoyant or,more often, approximately neutrally buoyant. Depending on the stage oftheir use and on the density of the surrounding seawater or fresh water,the fact whether or not a line is positively, neutrally buoyant ornegatively buoyant also depends on the density or densities of materialsused, materials contained, including fluids contained inside a line orlines. Many materials used degrade and absorb water while in service,accordingly, it is a common practice to supply any buoyant devices aswell as any devices desired to be approximately neutrally buoyant withsome excess of positive buoyancy.

Catenary equations typically approximate well shapes of mooring linesand flexible lines like hoses, flexible pipe, cables and umbilicals. Theapproximation involved is due to neglecting any bending stiffness of thesaid line or the said line segment. In addition to these, entire SCRlines of the simple (free hanging) configurations as well as for examplelazy wave SCRs are well approximated with catenary line equations indeep water, because in the said conditions bending stiffness of even arigid metal line is negligible in comparison with the scale of thestructure deployed. These include all configurations known of saidflexible and said rigid lines used in offshore engineering, some ofwhich are described by Baritrop¹.

With regard to the In-Plane (IP) shapes of the catenaries, for lineswith distributed weight and buoyancy, (as it follows from the catenaryequations) it is noted, that:

-   -   negatively buoyant catenary segments have their curvature        ‘bulging’ downwards,    -   neutrally buoyant or near vertical lines are well approximated        with straight lines,    -   and positively buoyant segments have their curvature ‘bulging’        upwards.

Discrete clump weights and buoyant connections (single clamps and buoys)IP result in local ‘sharp’ points or ‘spikes’ on catenaries, whereas:

-   -   Downward spikes occur at negatively buoyant devices;    -   No spikes are present at neutrally buoyant devices;    -   Upward spikes occur at positively buoyant devices.

Three dimensional, real catenaries have their shapes also modified inthe Out-of-Plane (OOP) direction due to drag in a current. The aboveobservations for the said IP shapes can be generalized to the shapemodifications OOP in the following ways:

-   -   Relative differences in drag between segments result in more or        less pronounced bulging with a uniform current, for segments        generating higher or lower drag, respectively;    -   Localized (discrete) drag devices that generate higher drag are        associated with sharper spikes.

Accordingly, in three dimensions, the combinations of the submergedweight (positive, neutral or negative) and drag forces are responsiblefor quasi-static shapes of catenary segments, while clump weights,tethered or clamped buoys are responsible for spikes in the shapes,because of the combinations of the weight, buoyancy and drag forces.Drag forces can significantly modify shapes of catenaries, depending onthe local strength of current (i.e. current velocity) and the dragcoefficient of any particular line segment or a device incorporated.Currents are seldom uniform along said lines. Typically both theirvelocities and directions vary along the line.

In addition to the above described, quasi-static effects of the weight,buoyancy, and current drag forces, which will be used to optimize theuse of this invention on particular examples, line dynamics plays asignificant part in the dynamic behavior of the said lines.

Dynamic effects on lines used in offshore engineering can be verycomplex. The said lines typically experience dynamic wave action thatdynamically modifies the said line configurations. Typically, the waveforces act as time variable drag forces and as time variable inertiaforces, approximately as described by the Morison Equation. These aremodified by the interactions between waves and currents that arecomplex, but for practical engineering systems it is usually acceptableto approximate the interactions by superposing currents with waveskinematically. Amplitudes of wave forces decrease along lines with thewater depth, which in deep water means the force decreases(approximately exponentially) to practically nil at deep water segmentsof the said lines. In addition to said wave forces, said lines are oftensubjected also to dynamic resonant excitations due to Vortex InducedVibrations (VIVs) in currents and waves. In addition to dynamic bendingof lines and to their fatigue loading, VIVs are also responsible,wherever they occur, for the increase in the quasi-static drag on theline.

It should also be stated, that many of the said lines are attached attheir top ends to floating structures that also move on waves. Themotions of the said structures add to the wave generated and othermotions of the lines, and they are directly transferred to said lines attheir top ends attached to said floating structure. All these motionsare transmitted dynamically as line deformation waves along the linecatenaries (straight shapes included) both up and down the catenarieswith differing velocities, dependent on a nature of the wave motiongenerated on the line.

In particular axial waves are transmitted along said lines very fast,approximately at the speed of sound in the materials used.

Catenary tension waves are also transmitted with similar velocitiesalong the line and they result in movements of the entire catenary,almost like a rigid body. A significant portion of the heave transferredto said line can result in motions of this kind and the deformationstravel along said lines slightly slower than the acoustic waves. Othermotions, together with the remaining part of the heave motion tend to betransmitted along said waves much slower as transverse deformationwaves.

Static and dynamic coupling exists between the torsion of the line andits bending wherever three dimensional bending occurs (torsion wavestend to travel along said lines faster than transverse deformationwaves). The latter interactions result in some redistribution of thecorresponding oscillation energies, however the amplitudes resultingtend to be small in practice and in most cases these phenomena can bedisregarded.

For said lines having multilayer structure, where different materialsare used in different layers the wave transfer velocities tend to differbetween layers, however the structurally dominant layers tend to controlthe motions.

All said waves traveling along said lines are subjected to reflectionson the lines whenever the mass and line directions change, as well theyare subject to dynamic interactions with the seabed. The quasi-staticand momentary dynamic shapes of catenary lines are tension controlled,and it is the property of the catenaries, that the effective tension isthe lowest at and near the touch down areas to the seabed (or at endsconnected to subsea structures), where the (effective)tension-controlled line stiffness is the lowest.

It is often the case that the effective tension near the touch-downbecomes periodically negative, making the line susceptible to localbuckling, which usually is not desirable and sometimes it is completelyunacceptable (example fiber-optic lines).

All riser and pipeline engineering codes that are also relevant toumbilical lines, cables, etc. recommend effective dealing with theproblem of the occurrence of negative dynamic effective tensions. Thesedecreases in the effective tension are often accompanied with dynamicreductions in the line radii of curvature. Bird-caging of umbilical orcable lines can occur, rigid or flexible pipes usually have somebuilt-in resilience, but complex local increases in fatigue damagetypically result. Often, in presently known designs it is difficult toincrease the effective tension and to increase the minimum dynamicbending radii to acceptable levels. Increasing the horizontal tension inthe catenaries, which increases also the quasi-static, average effectivetension at the touch-down in many known designs is known to often makethe dynamic effects described above even worse.

It is noted that the said effective tension is a physical valueresponsible for the line shape and buckling behavior for lines thatinclude fluid contained pipes, as described by Young and Fowler².Internal fluid pressures inside a rigid or flexible pipe, as well aspressures inside umbilical tubes, together with the external hydrostaticpressure in the surrounding water affect the actual (wall) tension inthe line or lines, whereas said effective tension governs the behaviorof the line. For some lines, like cables, electrical umbilicals or solidrods, effective tension and the actual tension are equal and they aresimply known as tension. However, with the above understanding the termeffective tension is used herein for all types of lines, wheneverrequired, because it is more general.

In particular, the said touch down zone line dynamics is in presentlyknown designs both significant and troublesome for simple, free hangingcatenary lines attached to floating structures. Examples of floatingstructures that are associated with the biggest motions are tankers(FPSs and FPSOs), particularly when they are bow or stern turret-moored.On such designs, all the risers, umbilicals, cables and mooring linesare attached to the turret, The motions of the FPSs and FPSOs aretypically the biggest at their bows and sterns, which are also typicallocations for turrets. However, many FPSs and FPSOs feature wide beamsin order to maximize their deck areas, and accordingly line topsattached to riser banks on vessel sides can also experience highmotions. Single Buoy Moorings (SBMs) and Semi-submersible vessels canalso transfer considerable motions to catenary lines. Top-end inducedmotions are typically smaller for articulated or compliant towers,Tension Leg Platforms (TLPs), SPARS, including Truss SPARS and otherdeep draught vessels, but they are by no means negligible.

In the presently known designs the most effective way of mitigating theproblem is to use one of the wave or ‘S’ configurations, as described byBarltrop¹.

The wave or ‘S’ configurations are sometimes unavoidable in shallowwater conditions and/or with strong variable currents. Because of largehorizontal motions of the vessel in these situations (that can be causedby waves, by variable currents or both), one of these configurations hasto be selected in order to reduce the maximum dynamic effective and walltensions in the catenary to an acceptable level.

In ultra deepwater conditions, the selection of for example lazy wavefor a flexible, cable or an umbilical line or for SCRs can also be thebest solution because of the line weight in its operational orinstallation configuration. In particular, at present, it might be notpossible to use larger diameter single pipe or Pipe-in-Pipe (PIP) SCRson some fields, where smaller diameter freehanging configurations are atpresent used. This is because the selection of a simple (freehanging)catenary configuration would have resulted in very high hang-off loads.These would have become even higher in a case of an accidental floodingof the line with seawater that might inadvertently happen duringinstallation or in operation. In such cases using a freehanging catenarymight be impossible, because the excessive hang-off load resulting mightbe too high to handle. Similarly, there might be no installation vesselavailable anywhere in the world, to handle such a heavy pipe during itsinstallation; or in particular to handle such a large diameter pipe orPIP, in a case of an accidental flooding with seawater. The feasiblesolutions in such cases would be to use wave or ‘S’ configurations,decrease loads with auxiliary buoyancy, or to use a larger number ofsmaller diameter lines that are lighter, so that the maximum tensionloads can be handled.

To summarize lazy wave, steep wave, pliant wave, lazy and/or steep ‘S’configurations according to prior art are used primarily because of twosets of reasons:

-   -   In shallow water in order to deal with large horizontal motions        of their top supports in waves and/or currents;    -   In ultradeep water in order to make large (tensile) loads        manageable;    -   An added advantage is some reduction in touch-down or bottom end        dynamics.        It is noted, that the average effective tensions at the top of        the lower negatively buoyant segments of lazy and steep wave and        ‘S’ configurations are typically of similar order of magnitude        as those at the line hang-offs. It is also noted, that for the        same reasons using modified wave or/and ‘S’ configurations        featuring more than one buoyant segment (buoy) are known. In        such cases the subdivisions of the negatively buoyant segments        of the catenaries is in known designs in segments featuring        comparable lengths and comparable maximum tension loads        resulting from similar design philosophy as that used for the        design of the single wave and/or ‘S’ configurations. This is        because of the same reasons of maximizing the flexibility of the        line (shallow water) or minimizing the maximum loads (ultradeep        water). However, it is noted that:    -   The use of the configurations in question, as implemented in        prior art, results in the increase of the suspended lengths used        (and in the corresponding increase in costs of the installation        that adds to the cost of the associated ‘additional’ hardware        used);    -   The selection of one of these configurations in prior art is        because of one of the underlying reasons listed above; in the        prior art these line configurations are not selected because of        the said added advantage. The reasons are economical, as        specified directly above.

Because of their higher costs, the energy industry tends to avoid usingsaid wave or ‘S’ configurations in conditions where simple catenariescan be made feasible. However, even for lazy wave, lazy S or compliantwave configurations, where partial dynamic decoupling can occur,Barltrop¹ states that touchdown line movements could also besignificant.

Another known way of obtaining a partial reduction in the said linetouchdown dynamics is a partial decoupling of motions by using a clumpweight low on a catenary. This method tends to be only partiallyeffective, because this makes the catenary above the clump weightsteeper and it can result in the heave motions being transferred moreeasily down to the location of the clump weight. It also increases boththe mass and the kinetic energy of the system moving, which would alsotend to work in the opposite direction to that, which is desired.However, due to the enhanced dynamic decoupling effect in this solutiontogether with careful tuning of the mass added and of its location tothe particular dynamic wave spectra prevailing on a field, a partialimprovement can be achieved.

BRIEF SUMMARY OF THE INVENTION

Undersea dynamic motion of a line, cable, pipe, riser or the like ismodified by the attachment of devices which locally change the buoyancy,the submerged weight, enhance the effective mass and modify drag dampingof the line at selected locations. Certain mass-enhancing devicesaccording to the present invention effectively add mass without beingparticularly massive themselves.

The size, shape, number and position of the mass/drag-enhancing devicesmay be varied to optimize the motion suppression effect. In particular,a novel line configuration is described in this specification thatoptimizes the use of buoyancy (depicted in FIG. 1), submerged weight,mass, added mass and drag in a particularly beneficial way.

The novel line configuration that optimizes the use of distributedsubmerged weight together with mass, added mass and drag is depicted inFIG. 2.

The said novel configurations depicted in FIGS. 1 and 2 aremodifications of a conventional, simple (free hanging) catenaryconfiguration, in particular, they can be used in new systems or theycan be retro-fitted on existing flexible, or rigid (steel, titanium,aluminum, etc.) free hanging catenary lines. The said novel lineconfigurations can utilize known types of buoyancy or can utilize novelbuoyancy shapes as also introduced in this specification and in thecommonly-owned patent application entitled “Dynamic Motion Suppressionof Riser, Umbilical and Jumper Lines” filed simultaneously. The novelfeature of the said configurations is that the locations along which thesaid devices are installed on the lines are located in the areas ofrelatively low effective tension. This includes the said installationlocations lying on the said lines in the vicinity of the seabed.

It is noted, in particular, that the novel configurations depicted inFIGS. 1 and 2 have been obtained by modifying simple, free-hangingcatenary line designs, without adding any line lengths in comparisonwith those of the original simple catenaries. These were done so inorder to demonstrate the suitability of this novel design to be used forretrofitting existing free hanging catenaries. Using the line lengthequal (or nearly equal) to that of a free hanging catenary is not,however, necessary to the practice of this invention.

However, the average effective tensions at the top of the line segmentsbetween the distributed buoyancy in FIG. 1 (5) or distributed submergedweight in FIG. 2 (5) in these novel designs are significantly lower thanthose at the line hang-offs.

Many implementations of the said novel buoyancy and weight clamp shapesaccording to this invention are also good Vortex Induced Vibration (VIV)suppressors. Accordingly, in addition to and instead of the use as wavedynamic suppressors they can also be used as primary or/and exclusiveVIV suppressors.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is an illustration of a catenary line (3) suspended from a bowturret (2) of an FPS or FPSO vessel (1). FIG. 1 depicts also a lineclamp of a known design (6 a) and eleven example implementations ofmotion suppression devices according to the invention (6 b through 6 l).The example devices shown (6) feature a positive overall line buoyancyalong the segment, where they are installed. The function of thecatenary line shown is immaterial. It can feature an SCR, a flexibleriser, an umbilical, a cable, a hose, a bundle of several similar ordifferent lines, etc.

FIG. 2 depicts a catenary line (3) suspended from a semi-submersibleplatform (1). FIG. 2 depicts also a line clamp of a known design (6 a)and eleven example implementations of motion suppression devicesaccording to the invention (6 b) through (6 l). The example devicesshown (6) feature a neutral or negative overall line buoyancy along thesegment, where they are installed. The function of the catenary lineshown is immaterial. It can feature a Steel Catenary Riser (SCR), aflexible riser, an umbilical, a cable, a hose, a bundle of severalsimilar or different lines, etc.

Optionally, the configurations shown in FIG. 1 and or FIG. 2 can alsofeature devices type (1 a through 41) mounted in the touch down region(7). The said optionally mounted devices in regions (7) could stretchbeyond the touch down points, where they would be in contact with theseabed (4), see FIGS. 1 and 2. The said optional devices installed likethose shown in regions (7) of FIGS. 1 and 2 could be installed on anyline configuration in order to mitigate the said line dynamics in thetouch down regions, including those installations where the elasticbehavior of the seabed is relevant to the design.

FIG. 3 shows a SPAR platform (1) having a catenary line (3 a) and atensioned line (3 b), both equipped with motion suppression devices (6).Segment (5) along the catenary line, to which devices (6) are attached,is selected by the designer for the purpose of motion suppression. Thecatenary line is suspended from a hang-off (2) and its lower end issupported by seabed (4).

FIG. 4 illustrates a TLP (1) having motion suppression devices (6)according to the present invention on both a catenary line (3 a) and ona tendon (3 b). Segment (5) along the catenary line, to which devices(6) are attached, is selected by the designer for the purpose of motionsuppression. The catenary line is suspended from a hang-off (2) and itslower end is supported by seabed (4).

DETAILED DESCRIPTION OF THE INVENTION

This invention allows the designer to locally fine tune several physicalproperties of lines, so that the desired motion suppression effect isachieved. The key line physical properties involved are the following:

-   -   Mass per unit length,    -   Added mass per unit length (described in terms of the added mass        coefficient),    -   Submerged weight and buoyancy per unit length,    -   Drag coefficient.

The above combined properties of the line, on which known or/and noveldevices are mounted combined with the properties of the said devices areof importance herein.

The above properties affect the statics and dynamics of said lines incomplex ways that have been outlined with regard to the prior artpertaining to the use of clump weights and buoyancy. This inventionextends the tools available to the designer by allowing more controlover the remaining said line physical properties, as well as moreflexibility in shifting between the added and the actual mass per unitlength as well more flexibility in utilizing the weight, the submergedweight and the buoyancy per unit length of the line.

In addition to extending the design tools, as already noted, thisinvention provides the designer with more opportunity to fine tune thedesign involving the said lines in offshore engineering.

The following general observation with regard to the properties utilizedaccording to this invention are noted:

-   -   The effects of the submerged weight and the buoyancy are static.    -   The effect of the drag is static, quasistatic and dynamic.    -   The effects of the added mass and the mass are dynamic.

Motion suppression involves dynamics, whereas the Newton's Second Lawapplies. Newton's Second Law implies in particular that greater massprovides greater ‘resistance’ to acceleration, and vice versa. Theaction of the hydrodynamic drag from the dynamic point of view issimilar, however, relative motions between a location on the line andthe surrounding mass of water matter:

-   -   wherever the line motion attempts to be faster than that of the        surrounding water, the line motion is decelerated;    -   whenever the line motion is slower than the relative motion of        the surrounding water, the line motion is accelerated by the        transfer of momentum from the water to the line.

It is contemplated that the dynamic interactions involving a motionsuppressor according to the present invention take place simultaneouslyin all three dimensions (and arguably in all six dimensions includingrotations that are also relevant to some extent) between the line andthe surrounding water, as well as due to the transfer of momentum andenergy along the line, in complicated ways. These involve propagation ofvarious kinds of the said waves, and their partial reflections at theends, at locations along the said lines as well as in interactions withthe bodies interacting, like the seabed, structures attached and thewater surrounding. The said ways are propagated along the lines in waysthat can be partly approximated as one dimensional—predominantly alongthe lines, but there are also important two dimensional effects thathappen independently in the IP and OOP directions, wherever the linedirection changes.

This invention utilizes the said four line properties as theysimultaneously affect said complex six, three, two and one dimensionalprocesses that are mostly dynamic and quasistatic. As the result ofutilizing the invention static, quasistatic and dynamic results areachieved, the primary objective being dynamic motion suppression.

The said dynamic motion suppression has the combined purpose as follows;

-   -   The reduction in the dynamic component of the effective tension,    -   The increase in the lowest values of effective tension anywhere        along the line,    -   The reduction of line susceptibility to global and local        buckling, including buckling resulting from local interactions        of different layers, components of layers involving the line        construction, if applicable,    -   The increase in the minimum dynamic radius of curvature anywhere        along the said line,    -   The reduction of the fatigue damage and associated increase in        the fatigue life of any line components, including those of the        internal line construction, if applicable,    -   The reduction in the range of variable stress components in the        said line, including stress components in different line        construction components, made of similar or largely differing        materials, if applicable,    -   The reduction in the line susceptibility to bird-caging.

For the purpose of this invention dynamic line excitations can bedivided into two categories:

-   -   Approximately periodic that can be well approximated with        regular, i.e. close to sinusoidal excitations, typically in one        to six degrees of freedom;    -   Transient Excitations, also typically in one to six degrees of        freedom.

Regular excitations of very long and/or highly damped lines, wheneverstanding wave patterns are not generated, are considered as transientexcitations for the purpose of this specification.

Real line excitations in offshore conditions typically combine both thesaid excitation categories. The said combination is typically non-linearand accordingly the load superposition does not apply in general,however, in many practical load scenarios it can be useful to consider alinear approximation of the dynamic system considered, which is asimplification of the real line and its dynamic loading.

Unless the line is very long or damping is very high, the said periodicexcitations often generate standing wave patterns on said lines. Alinear approximation of the standing wave component of the loading of aline allows the designer to use the following simple guidelines indealing with the said standing wave loading of the said line:

-   -   Maximize drag per unit length along the line segment where the        said devices are installed;    -   Minimize the combined mass and added mass per unit length along        the line segment where the said devices are installed;    -   Depending on whether the design objective is to reduce line        dynamic motions within line regions where the said devices are        or are not installed:        -   Minimize the combined mass and added mass per unit length            along the line segment where the said devices are installed,            in cases where the objective is to reduce the line dynamic            motions along the bare line segments;        -   Maximize the combined distributed mass and added mass per            unit length locally, along the line segments, where the said            devices are installed, in cases where the objective is to            reduce the line dynamic motions of the said line segments            where the devices are installed.    -   If feasible, tapering of combined line properties should be        considered whenever they change; these include in particular        combined bending stiffness of the line and devices added (i.e.        use of bending restrictors and/or bending stiffeners, and/or        stress joints and/or tapered or stepped transition joints).        Tapering other properties like the submerged weight, buoyancy,        drag, mass and added mass might also be worth considering.        Varying any properties can be achieved in particular by varying        the number of devices used per unit line length and/or by        modifying physical properties of the said devices.

In all the said cases, the designer needs to consider in detail theparticular dynamic and hydrodynamic characteristics of the line beingdesigned, the dynamics of any structures or other bodies relevant aswell as the character of loading and the way it is propagated along theline. In particular, the line drag, mass and/or added mass per unitlength can be utilized to suppress motions. Tapering of the said lineproperties can be also utilized and in general case the design needs tobe evaluated and optimized using mathematical modeling. Commerciallyavailable line modeling programs are very useful for this purpose andthey allow to model both the standing wave and transient load component.

The design evaluations and/or optimizations generally involve a numberof design load scenarios (or loadcases) and the design and/oroptimizations are performed in an iterative process (essentially bytrial and error) until the design objectives are achieved or until theoptimal system configuration is found.

Referring now to FIGS. 1 through 4, a variety of devices according tothe present invention are illustrated. These devices are mounted on,rigid (steel, etc.), flexible and tensioned risers, umbilicals, cables,tendons or the like (hereinafter “line”). The devices shown are used fortuning locally the overall line submerged weight (including thebuoyancy), mass per unit length, added mass per unit length, drag andbending stiffness of an associated line segment.

FIGS. 1 (6 a) and 2 (6 a) depict motion suppression devices of a knowndesign are installed concentrically on lines 3. The devices shown areeffectively mechanical clamps attached to the lines using any knownmeans, (utilizing bolts, tape straps, adhesives, welded in place, etc.).Motion suppression devices of known design may feature a large varietyof shapes and mounting arrangements, the split-cylindrical one shown forexample is the most common one.

FIGS. 1 (6 b) through (6 l) and 2 (6 b) through 4 (6 l) depict exampleembodiments of the invented shapes. Attached to the exterior surface ofthe clamps are external plates, which may intersect at a large varietyof angles (including right angles).

The said plates act to increase the overall added mass and hydrodynamicdrag of the devices to which they are attached, and accordingly toincrease locally the added mass per unit length of the line, and toincrease locally the selected drag force components per unit length ofthe line, including all drag force components.

The size and shape of the novel devices are designed to increase theadded mass and the hydrodynamic drag of the line to the arbitrary levelrequired by the designer. The increase in the added mass is because ofthe dynamic pressure distribution on all external surfaces (includingthe plates) of the device, whenever the motion of line and the devicechanges relative the surrounding fluid (relative acceleration). Thismanifests itself as if an additional mass of water were entrapped, andmoved together with the line and the device. The actual mass, weight,submerged weight and buoyancy of the device the plates included, alsocontributes locally to the actual mass, weight, submerged weight andbuoyancy per unit length of the line.

It is noted that the example embodiments of the novel devices depictedon the said FIGS. 1 (6 b) through 1 (6 l) and 2 (6 b) through 4 (6 l)are examples only that illustrate the novel design principle involved.The novelty involved is functional and the actual number of realizationspossible is much greater than it is practical to depict on drawings inthis specification. However, selected design options and design featuresare discussed briefly further in this specification.

The present invention provides a riser, umbilical, jumper, cable andhose motion suppressing arrangement for use primarily but notexclusively in deepwater. This invention pertains to lines includingflexible risers, umbilical lines and cables including any combination ofelectrical lines, hydraulic lines, pneumatic lines, fiber-optic lines,telecommunication lines, acoustic: lines and any other kind of linesthat are used in offshore technology. This invention also pertains tohose lines, jumper lines, Steel Catenary Risers (SCRs), tensionedrisers, including freestanding tensioned risers and hybrid riser towers,Said invention also pertains to hybrid risers and umbilical lines thatmight include any combinations of flexible and rigid (steel, titanium,aluminum and any other metal) lines, including tendons, and tethers. Allsaid lines and other similar lines that are used in the offshoretechnology are referred herein as lines, which for the purpose of thisspecification include all types of lines identified herein and all typesof bundles of lines, including riser bundles and pipeline bundles inoperation, during their transport and installations. These also includeany configurations of the said lines used offshore, inshore and ininland waters. High curvatures of said lines on some configurations,together with their low slopes may be utilized, see simple catenaryline, FIG. 1. The original line configuration may or may not bemodified. Known motion suppressing device designs can be used, see FIGS.1 (6 a) and 2 (6 a). Because of the low slope on some configurations(line parallel or nearly parallel to the seabed), said motionsuppressing devices can be installed on arbitrarily long line segments,which can be designed as long as necessary in order to achieve thedesign objections required. Novel, drag and added mass enhancingdevices, see FIGS. 1 (6 b) through 1 (6 l) and 2 (6 b) through 4 (6 l),effective in all directions can be used to increase the suppressioneffectiveness and/or in order to reduce the number of devices used or toreduce the lengths of the motion suppressing segments, This invention issuitable for use on new designs and it is also suitable for retrofittingon existing, already installed lines.

This invention is illustrated further below in examples of use of theinvented device for a motion suppression of simple (free hanging)catenary configurations of risers, cables or umbilical lines, see FIGS.1 and 2. Similar devices would also be effective while used in variouslocations of other configurations on other types of lines, in particularon lazy wave, pliant wave, and/or steep wave configurations as describedfor example by Baritrop¹.

Two similar example implementations, shown in FIG. 1, of this inventionare illustrated herein. A similar implementation of this invention usingmotion suppressing devices according to this invention having positivesubmerged weights is shown in FIG. 2. These examples are used herein todemonstrate this invention and to highlight the design reasoninginvolved. All three examples described herein involve optimizations ofthis invention for modifications of the simple catenary lineconfigurations according to this invention. Simple catenaryconfigurations are those that experience dynamic touch-down conditionsthat are the most difficult to deal with, at least in deepwater.

The original simple catenary line according to a known design and bothmodified configurations optimized according to this invention used thesame flexible line characteristics, including the same submergedweights, the same axial and bending stiffnesses as well as the sameoutside diameters and allowable minimum radii of curvature in dynamicconditions. All these parameters typically vary in wide ranges dependingon particular design objectives required. Similar results to thosedemonstrated by mathematical modeling of the known design, and the newdesigns according to this invention can be obtained for other linescharacterized by other sets of design parameters. In particular, the twoexamples of the designs according to this invention used herein for thesake of a demonstration depicted in FIG. 1, were very similar, they hadexactly the same quasi-static real catenary configurations of a riser oran umbilical, which are depicted in FIG. 1. In order to demonstrate,however the design advantages of this invention that occur even withwidely varying technical characteristics, the drag coefficients and theinertia coefficients of the short, close to slightly positively buoyantsegments (5) added to the catenary close to the touch-down differedconsiderably.

For the sake of the said examples the top ends of the line (3) wereattached to a bow turret (2) of a floating tanker vessel (1). The seabed(4) was assumed to be horizontal. For the sake of the examples depictedin FIG. 1, a distributed, slightly positively line segment (5) wasutilized as an implementation of the invented arrangement in order tosuppress line dynamics in the touchdown zone.

It is noted, that the devices designed according to this invention addedto suppress motions could be positively buoyant (see FIG. 1), neutrallyand negatively buoyant (see FIG. 2), could be distributed and could beplaced in discrete locations, depending on the design objectives of thedesigner, including but not being limited to the degree of modificationof the variations of average components and to the extents of variationsin the dynamic components of technical parameters, for example the saideffective tension and for example the said minimum radius of curvature.The devices installed on the lines should preferably be located withinthe lower ⅜ of the line suspended length, but they can be installed aslow on the lower ⅓, ¼ or even ⅛-th of the line suspended length from thelocation of the touch down or from the location where the line isconnected to its lower end attachment.

The said original and both the said modified catenary configurations inthe examples shown on FIG. 1 use the same top of the line departureangles from the horizontal. While one uses the catenary lineapproximation of a real line shape, it is noted that for a given waterdepth, with a given top line support elevation and a given average slopeangle of the seabed the IP shape of an ideal catenary line is uniquelydefined and it is described with an algebraic mathematical equationinvolving a hyperbolic function cosh. Accordingly, the top departureangle is a convenient parameter to describe shapes of real lines usedoffshore.

It is also noted that said top catenary angles used in offshoreengineering vary in a wide range, depending on the water depth and setsof other parameters that depend on particular design objectives, typesof the surface structures used and their motion characteristics, ifrelevant, types of lines used, configurations of other, neighboringlines that need to be cleared, etc. In particular, on the high side itis common to use in deepwater, umbilical line nominal departure anglesof close to 88° and to 89° from the horizontal, and both values up to90° and much lower values are assumed by line catenaries used on severalGulf of Mexico Truss-SPAR platforms due to low and high frequencymotions as well as due to shifting the platform mean location betweenvarious design parking positions. On the lower side it is mentioned thatfor example SCRs in not very deep water can use top departure angleslower than 65° or even lower than 55° and many mooring lines used havenominal top departure angles close to 45° and lower in deep water, andeven considerably lower in less deep water. This invention can be usedwith many types of lines in many configurations having any top departureangle selected from a wide range by a designer.

This invention involves the design optimization process that extendsbeyond usual known design considerations combined with providingadequate, novel means to achieve the design level of motion suppressionin key design areas of lines used in offshore engineering. In order toachieve a desired level of motion suppression according to thisinvention, drag damping and added mass are utilized. For the examples ofthe simple (free hanging) catenary lines demonstrated herein (FIGS. 1and 2), the key regions of interest are the touchdown zones. The saidproperties of catenary lines that were already highlighted herein areutilized in a novel way according to this invention in order to achievethe design objectives required.

In particular, it is desirable to utilize drag and added mass along aline to an extent required. Near the touch down area, a simple catenaryhas its maximum design curvature. This makes the selection of the areaadjacent to the touch down particularly effective in the maximizing ofthe motion decoupling process. In particular, using buoyancy or/andapproximately neutrally buoyant drag and added mass enhancing devicesaccording to this invention directly adjacent to the touch-down area areparticularly advantageous novel ways in achieving motion suppression.That is more effective than using say a traditional lazy waveconfiguration just in order to deal with the touchdown dynamics, whenthere is no other, governing reason for selecting a lazy or pliant waveor a lazy S configuration.

In particular, it is noted, that in the touch-down area, the catenaryline has naturally a small slope angle, in addition to the largecurvature that is utilized to enhance decoupling. Clamping buoyancy on aline increases its drag and its added mass. Accordingly, it is naturalto utilize the small slope together with the neutral buoyancy of a linesegment that can be extended almost indefinitely to a segment lengththat is required to achieve the motion suppression desired. In order tocompensate for the natural aging of most buoyant materials used, thismeans in practice a slight overall positive buoyancy of the line segmentadded. The additional advantage of the slight positive buoyancy is, thatif desired so, the slight original downward slope of the catenary in thetouch-down zone can be compensated with slight positive buoyancy, sothat the average added segment slope can be modified to any desireddownward, horizontal or upward value required, so that there is nophysical limit to the selection of the length of that novel segmentrequired according to this invention. Mathematical modeling proved, thatwhile using buoyancy elements of known design, FIG. 1 (6 a), which arefeatured with traditional values of the drag and inertia coefficients,effective tension compression (i.e. negative values of the effectivetension) was removed for the line example depicted in FIG. 1, in spiteof extreme seastate conditions used. Neither of the above was achievablewhile using the known simple catenary configuration for the tankervessel motions and the typical line characteristics used. In addition tothis, the minimum values of the radius of curvature were increased tothose considerably above the allowable value. It is understood here thatthe inertia coefficient incorporates the added mass coefficient and alsoaccounts for the Froude-Krilov forces on a body considered.

It is noted, however, that for the configuration, according to thisinvention depicted in FIG. 1, but utilizing buoyancy clamps of knowndesign, FIG. 1 (6 a), significant tensile (positive) dynamic componentswere present in the values of the effective tension and in the values ofthe radius of curvature. It is also noted, that in a similar modelingexercise with a short buoyant segment located slightly higher on thecatenary, it was not possible to keep the effective tension positivethroughout the modeling time span (irregular sea of pre-definedduration). However, by utilizing distributed buoyancy according to thisinvention as shown for example in FIGS. 1 (6 b) through 4 (6 l), theminimum radius of curvature in the dynamic line motion was increased toan acceptable value, see below.

The second example design according to this invention presented hereinutilized drag and added mass enhancing devices according to thisinvention, like those depicted in FIGS. 1 (6 b) through 4 (6 l). Theshape and the size of these devices can be designed to increase the dragand inertia coefficients considerably, see FIGS. 1 (6 b) through (6 l)for some examples. In general, the larger the dimensions of the shapesused, the larger the drag and inertia coefficients will be. Theseallowed significant improvements in the effectiveness of the drag andadded mass suppression invented. It is noted in particular, that thelocal discrete or distributed increase in the added mass, could intheory, be as effective in decoupling motions as using a clump weight,however, the added mass of water does not have the undesirable effectsof making the catenary steeper and transmitting the heave motions moreefficiently to the lower regions of the line. Increasing the drag forceslocally results in additional damping, i.e. dissipation of theoscillation energy transmitted along the line and stored in thevibrating system.

The use of the enhanced drag and enhanced added mass devices in thesecond example described herein, like the examples shown in FIGS. 1 (6b) through 4 (6 l), resulted in additional large reductions in thedynamic components of the effective tension and increases in the minimumradii of curvature. In fact, the modeling demonstrated that the lengthof the modified segment (5), as shown on FIG. 1, could have been reducedconsiderably in comparison with that used and the improvements achievedwould still be considerable.

Several examples of the drag coefficient and the inertiacoefficient-enhancing shapes are depicted in FIGS. 1 through 4, but manymore are possible and can be used in implementing this invention. Thereare so many configuration selection possibilities that it would not havebeen practically possible to demonstrate them all on drawings or tofully describe all the possibilities. Accordingly, a general descriptionfollows that highlights the outline of the possibilities existing. Inparticular any combinations of triangles, squares, rectangles, otherpolygons like that shown for example in FIG. 1 (6 f), circles, ellipses,ovals, star-like shapes and many others in absolutely arbitrarycombinations can be used.

The design arrangement according to this invention of the shapes usedfor the drag and added mass enhancements is important. Because said linemotions in the touch down regions are three dimensional, or to be moreprecise five dimensional if one adds rotations IP and OOP, the shapesused according to this invention provide the drag and added massenhancements that are simultaneously effective in more than onedirection and preferably in any three directions, that would be affectedapproximately similarly to three mutually perpendicular directions. Inparticular, the drag and added mass enhancements according to thisinvention are recommended to be effective in the axial direction andsimultaneously in both IP and OOP directions of the catenary. However,any other selection of directions can be used if that selection has asimilar effect. Numerical modeling shows that drag enhancing only in theaxial direction, for example that suggested by U.S. Pat. No. 4,909,327,enhancing drag in the axial direction of a line is not very effective.

The areas and the aspect ratios of said devices that enhance the dragand added mass in differing directions need not be the same, in fact inthe general case they would be different, see FIGS. 1 through 4. Theaspect ratio is defined herein as the square of its maximum dimensionpresented to the flow divided by the surface area of a given shapepresented to the flow along the mean normal vector to the surface of theshape (this is equal to the ratio of the effective span length of theshape to its mean chord length). For instance, for a square and arectangle the said maximum dimensions are the lengths of theirdiagonals.

Three dimensional arrangements of the drag and added mass enhancingfeatures can be very complex. In particular, in addition topredominantly planar appendage shapes that are shown in FIGS. 1 (6 b)through (6 l), curved shapes, in general featuring both curvatures andtwists can also be used. For example, FIGS. 1 (6 e) and 1 (6 f) depicthelical strakes. The shapes can feature smooth or rugged edges, likethose shown for example in FIG. 1 (6 d), FIG. 2 (6 d and 6 i through 6j), FIGS. 3 and 4 (6 d and 6 i through 6 j). Any of the added mass anddrag enhancing devices described herein can also feature drag and/oradded mass enhancing holes and/or slots that could in some situations bemore effective than solid shapes, similarly to holes and/or slots thatare used in the designs of some parachutes.

The use of the drag and inertia coefficient enhancing shapes accordingto this invention provides a designer with several additional designoptimization tools according to this invention:

-   -   Selecting the actual shapes and the design parameters of the        motion suppressing shapes, while having additional design        philosophy aspects in mind, for example the OOP shape of the        catenary in case of a significant cross-current, VIV        suppression, etc;    -   Selecting the appropriate shape dimensions for the level of        suppression required;    -   Balancing between the effectiveness of the shapes, buoyancy,        submerged weight used, the length of the motion suppressing        segment and/or the number of said suppressing devices used, etc.

Three important design philosophy aspects might need to be considered inthe design of the drag and added mass motion suppressing arrangementaccording to this invention. They are both related to a particularcurrent profile.

-   -   The first one regards the way drag in a current affects the        shape of the design catenary;    -   The second one is related to the way any design modifications        according to this invention would affect VIVs of the line, if        relevant;    -   The third is that the drag and added mass enhancing devices        described herein can be used anywhere on lines also with the        primary purpose of VIV motion suppression.

On most field locations currents tend to decrease with the water depthand they tend to become even weaker near to the seabed. These tend to bebeneficial, because local drag increases would tend to result in smallerdistortions of the line shape, than those that might occur for examplein lazy or pliant wave configurations. However, the above is not alwaysthe case, on some location's bottom currents could be particularlystrong. In such situations these aspects need to be included in thedesign process and the locations of the drag and added mass motionsuppressing arrangement might need to be moved higher along thecatenary. It is noted, however, that this does not necessarily need tobe the case, the dissipating effectiveness of hydrodynamic drag improveswith increasing current. The effectiveness of the added mass suppressingcomponent in a current might require additional consideration anddesigner's attention in a case of a current. The actual shapes used forthe suppression enhancement might be of importance in this context.

With regard to the VIV potential, it is noted that in general both theuse of buoyancy of known design (FIGS. 1 (6 a) and 2 (6 a) and/or thathaving invented shapes (FIGS. 1 (6 b) through 1 (6 l) and 2 (6 b)through 4 (6 l) for additional motion suppression will tend to improvethe VIV situation, because of the local decrease in the reducedvelocity, due to the increase in the hydrodynamic diameter. Theadditional improving effect of the increase in the hydrodynamic diameterwould in most cases be increased drag damping, which would tend toincrease the damping of the whole dynamic system. In fact, unless thecurrent is very strong the designer of a system according to thisinvention has additional tools to reduce the VIV susceptibility of theentire dynamic system. The additional tools involve the freedom to usebeneficial hydrodynamic diameter in order to reduce locally the reducedvelocity, use of beneficial shape configuration to increase thehydrodynamic damping in the system, as well as shaping the dampingappendages so, that additional vortex generation suppression results.The latter could include adding helical pitch to the design of theshapes, see for example FIGS. 1 (6 e) and 1 (6 f), in order to providethem with added vortex suppression effectiveness, using rugged edgeslike those depicted for example in FIGS. 1 (6 d), etc. The issue of theadded mass could be more complicated in case the invented suppressionarea increases the VIV energy of the system. In such cases added masscould be even negative and additional, more complex optimizationconsiderations could be necessary. Accordingly, the general guideline isto try to reduce the reduced velocity in the regions designed for themotion suppression and consequently to enhance their effectiveness bothin the wave oscillation frequency range and in the VIV frequency range.

It is noted that known strake designs used in order to suppress VIV(like those shown for example in U.S. Pat. Nos. 6,695,540B1 or6,896,447B1), would in principle have different geometrical featuresthan strakes designed to enhance the drag and added mass according tothis invention. Many geometries of VIV suppressing strakes are used inthe offshore technology, some had never been model tested before theinstallation in the ocean. However, those strake designs that arejustified by extensive model testing programs and many years of researchtend to have strake height to root diameter ratios of the order of 25%or lower. Usually, three strakes are arranged on the circumference.Typical configurations have pitch of the order of 17 times the rootdiameter.

However, some European tests recommend strakes of the pitch three tofour times smaller. These tend to result in less effective VIVsuppression, but the drag of the line tends to be smaller. Generally,VIV designers try to optimize the VIV amplitude reduction effectivenesswith minimizing the hydrodynamic drag of the strakes. These objectivesare different from those desired herein, and accordingly the designsresulting would preferably differ. In particular, if helical strakes areutilized according to this invention, they would preferably be alsofitted with axial drag increasing plates, like those depicted forexample in FIGS. 1 (6 e) and 1 (6 f) that are not used on VIVsuppressing strakes. In addition to this, it is noted that maximizingthe drag and the added mass would tend to favor higherheight-to-root-diameter ratios. In particular, those strakes shown inFIGS. 1 through 4 have the height-to-diameter ratios on the order of50%, and even higher fins could be used.

The strake heights and other features would typically be affected alsoby other considerations like a manufacturing process used, economicconsiderations, installation configuration limitations, etc. that mighttend to reduce the height of the strakes used in any particular design.Also, drag is better enhanced if more than three fins are used on thedevice circumference, in particular the example depicted in FIG. 1 (6 f)uses for sake of instance four fins, while that of FIG. 1 (6 e) usesonly three fins; using other numbers of fins is also feasible.

It is noted that other shapes according to this invention also have highVIV suppression effectiveness, in particular the shapes utilizing ruggededges. These shapes can feature rugged contours, with or without helicaltwist. Rugged contours result in forcing wake vortices to be shed atparticular lengths, which can be varied by the designer by selectingirregular ruggedness patterns or/and by mounting devices on lines atirregular intervals.

Arbitrary geometrical shapes can be used in many implementations of thisinvention. The said shapes can intersect at arbitrary angles, includinga wide range of acute angles and right angles. It is understood herein,that any flat or curvilinear surfaces intersecting at other than a rightangle will define at least two values of angles, the governing one ofwhich will be an acute angle and the other one being 180° minus the saidacute angle.

It is also noted that manufacturing and installation limitations canalso limit the size of any shapes used. In general they can have simpleconstruction or they can be strengthened with ribs, they can use fiberreinforcement technology, they can utilize strengthening brace members,etc., none of which are shown for the sake of simplification in FIGS. 1through 4.

In particular installation or transport requirements would often affectthe detailed design of the said novel shapes. In particular, thedesigner might decide to provide the said devices with additionalstrengthening, for example additional ribs or braces that would provideadditional protection or/and increase the bearing strength of the saiddevices, with regard to contact with external bodies. This could bedemanded by a need to withstand contact loads with other equipment forexample with a stinger of an installation vessel, with a ramp, with aj-lay tower components, a contact with a beach during launch, aninteraction with the seabed during a bottom tow, in the touch down areaduring operation, etc.

It is noted that the devices used might use split clamp design(symmetrical, or asymmetrical, including designs that are split on oneside), the details of which are also omitted for clarity from theisometric views presented in FIGS. 1 (6 b) through 1 (6 l) and 2 (6 b)through 4 (6 l). It is noted that any materials and constructionprinciples used in subsea engineering are suitable for use to design andto build said drag and added mass enhancing devices. Devices of the sameand of mixed technical features can be used on the same line, if sorequired. They can be mixed along the line, or in particular theirtechnical characteristics including the shapes, material densities, dragcoefficients and added mass coefficients can be modified gradually alongsaid line or lines in order to achieve any particular design objectivesrequired. Optimizations using mathematical modeling are useful and costefficient, however, specific model testing programs would be a usefuldesign optimization tool.

It is noted that with some sets of design requirements including thedesign requirements on the line properties, the met-ocean conditions andthe characteristics of the top support structure (i.e. vessel, buoy,etc.), it might be relatively easy to configure the design arrangementaccording to this invention, so that the dynamic compression is removedor reduced to a desired level. However, particularly in ‘morechallenging’ irregular sea conditions it might be more difficult tooptimize the design to limit dynamic bending as well.

In cases where the reduction of the minimum radius of curvature beyondthat easily achievable by using the said dynamic decoupling arrangementaccording to this invention alone is less easy than dealing just withdynamic compression, it might be advisable to use also traditionalstress joints (with uniform or varying properties, including tapered andstepped stress joints), bending restrictors or bending stiffeners, etc.,as desired, at one or both ends of segments where the added mass and/ordrag properties and/or submerged weight (buoyancy included) aremodified.

Bending stiffeners and/or bending restrictors and/or uniform and/ortapered stress joints can be used with segments having constant or/andvariable said modified line properties along the segment length. Inparticular, tapering of the line properties towards one or both segmentend(s) can be utilized. What is meant here, is also using mass, addedmass, drag coefficient, submerged weight, buoyancy, etc. that arevariable along the line, according to this invention, alone or/andtogether with traditional means to govern bending, like those providedby traditional stress joints, tapered transition joints, bendingstiffeners, bending restrictors, etc. These include combining the saiduniform or said variable line properties according to this invention,with those of the said traditional bending control devices. The saidcombining can be performed so, that:

-   -   The said bending control devices can be installed at an end or        at both ends of the segment(s) having modified properties,        according to this invention;    -   The said segment(s) having modified properties, according to        this invention can be simultaneously featured with modified        bending properties, so that they can also perform like a        traditional bending restrictor or bending stiffener;    -   Stress joints and/or stepped and/or tapered transition joints        can be used at the locations with modified hydrostatic and/or        hydrodynamic line properties according to this invention and/or        they can be used at adjacent location or locations.

The physical properties of line appendages, whether of known or noveldesign are determined in the design process in the usual way using thedensities of the materials selected and their dimensions, which resultin volumes that can be calculated. The said physical properties include:

-   -   mass of the said appendages per unit length of the line,    -   weight in air of the said appendages per unit length of the        line,    -   buoyancy of the said appendages per unit length of the line,    -   submerged weight of the said appendages per unit length of the        line.

Of course, the said submerged weight is equal to the difference betweenthe weight and the buoyancy.

The added mass per unit length and the drag coefficients of appendagesof known design as well as those of some of the isolated shapes added tothe appendages of novel design presented herein are known (or in thelatter case they could be known approximately) from technicalliterature, like DNV CN30.5³. However, in most cases, the remaininghydrodynamic properties of the said appendages:

-   -   the added mass of the said appendages per unit length of the        line (the added mass coefficient),    -   the drag of the said appendages per unit length of the line (the        drag coefficient),        are determined from hydrodynamic model tests. The hydrodynamic        model tests would in many cases include some variations of the        geometries of the appendages tested.

Knowing the above properties, the designer refines the design of thedynamic motion suppression of the line using mathematical modeling. Thisis performed using specialized computer programs (including thosecommercially available) or equivalent (the ‘equivalent’ might includecustomized databases prepared previously using mathematical modeling,etc.). The refining process typically involves parametric studiesincluding the variation of the said line property parameters specific tothe specific design criteria of the line until the desired or optimalline suppression design is achieved. The said design criteria of theline would typically include for example: water depth; base lineproperties and geometry; platform, buoy, etc motions; wave climate,current profile; clashing potential with other lines and equipment; etc.

In order to suppress the line dynamics according to these guidelines,the designer typically maximizes the drag along the line. With regard tothe line effective mass per unit length, the general guideline is tomaximize it to the extent feasible in the regions where the greatestdynamics occurs, in particular the transverse line dynamics. However,the limitation on the said increases in the effective mass by using saiddevices type (FIG. 1-6 a through 4-6 l) tend to increase the standingwave dynamics along the bare segments of the line, where relevant. Thedesigner needs to fine tune the design, while taking into account theabove counteracting tendencies. Important additional design tools aretapering the said line properties, including using bending stiffeners,restrictors, stress and transition joints, etc. as described herein.

In some cases variations of the design process outlined above can beselected instead, while still including in principle the major actioncomponents described above. This could include for example refining thesaid line properties in the preliminary design process and subsequentlyusing hydrodynamic model testing in order to refine the specific saidline appendage properties.

Whichever design ‘flowchart’ is used, the design process typicallyincludes several design iterations. Model testing iterations might alsobe required, a tendency is to keep a number of these to a minimum.

In addition to the above mentioned, the design iterations typically dealwith a number of usual design issues like static and dynamic positiveand negative effective tension, allowable bending moments, minimumradius of curvature, maximum dynamic stresses, fatigue, as alreadydescribed herein, etc.

This invention involves:

-   -   Dynamics decoupling, damping and added mass enhancing        arrangement,        -   including a single device,        -   and also including a system of multiple devices,        -   and also including any plurality of systems of such devices,    -    which affect line dynamic motion in marine engineering.    -   Said line including: any flexible riser, any umbilical, any        cable, any mooring line, any tether, any tendon, any hose, any        jumper, any tensioned riser including free standing risers, any        hybrid riser tower, any steel catenary riser, any rigid riser        made of another metal, including any plurality of metals and        alloys, including titanium and aluminum.    -   Said line being made of any other natural, and said line being        made of any other man made rigid material, and said line being        made of any man made flexible material, and said line being made        of any other natural rigid, and also including said line being        made of any other natural flexible material, and said line being        made of any combination of manmade, natural, flexible and rigid        materials.    -   Said line having predominantly one kind of construction, and        including said line being of hybrid nature and incorporating        said lines having differing line constructions, including line        segments having differing line constructions.    -   Said arrangement in particular:        -   utilizing buoyancy,        -   and also said arrangement utilizing submerged weight,        -   and also said arrangement being approximately neutrally            buoyant    -    Whereas any of said positively, negatively and neutrally        buoyant devices utilizes also:        -   its own mass        -   in addition to its added mass        -   and in addition to the hydrodynamic drag it generates during            its dynamic motions;    -    and also said arrangements in particular        -   utilizing buoyancy,        -   and also said arrangements utilizing submerged weights,        -   and also said arrangements being neutrally buoyant.    -    Whereas any of said positively, negatively and neutrally        buoyant devices, including arbitrary combinations of said        positively, negatively and neutrally buoyant devices,        -   utilize also their own mass        -   in addition to their added mass and        -   in addition to the hydrodynamic drag they generate during            their dynamic motions.    -   Said arrangement utilizing natural catenary properties, said        properties including in particular:        -   relatively low average effective tension at and near the            seabed end of said line catenary        -   and also relatively low-average effective tension directly            above buoyant segments and buoyant arches and buoys of lazy            wave, pliant wave, steep wave, lazy S, steep S, Chinese            Lantern.    -   Said catenary line properties, which for said particular line        configurations incorporating simple, free hanging catenaries,        lazy wave and pliant wave catenaries and lazy S catenaries,        might also include:        -   a relatively high curvature        -   combined locally with said relatively low average effective            tension.    -   Said catenary line properties, which for said particular line        configurations incorporating simple, free hanging catenaries,        lazy wave and pliant wave catenaries, and lazy S catenaries        might also include:        -   a relatively low line slope with regard to the slope of the            seabed,        -   combined locally with said relatively low average effective            tension.    -   Said positively buoyant device, and said neutrally buoyant        device, and said negatively buoyant device, including any        plurality of said positively buoyant devices, and said neutrally        buoyant devices, and said negatively buoyant devices, which        utilize:        -   any of said catenary line properties alone, and        -   which utilize any plurality of said catenary line            properties.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the mass of said device,        -   including being utilized together with masses of any            multitude of said devices    -    in order to reduce the dynamic component of said effective        tension, including and excluding any negative component of said        dynamic effective tension at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the buoyancy of said device,        -   including being utilized together with buoyancies of any            multitude of said devices    -    in order to reduce the dynamic component of said effective        tension, including and excluding any negative component of said        dynamic effective tension at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   approximately neutral buoyancy of said device,        -   including being utilized together with approximately neutral            buoyancies of any multitude of said devices    -    in order to reduce the dynamic component of said effective        tension, including and excluding any negative component of said        dynamic effective tension at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the submerged weight of said device,        -   including being utilized together with submerged weights of            any multitude of said devices    -    in order to reduce the dynamic component of said effective        tension, including and excluding any negative component of said        dynamic effective tension at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the drag force on said device,        -   including being utilized together with drag forces on any            multitude of said devices    -    in order to reduce the dynamic component of said effective        tension, including and excluding any negative component of said        dynamic effective tension at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the added mass of said device,        -   including being utilized together with added masses of any            multitude of said devices    -    in order to reduce the dynamic component of said effective        tension, including and excluding any negative component of said        dynamic effective tension at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the mass of said device,        -   including being utilized together with masses of any            multitude of said devices    -    in order to increase the dynamic minimum in the variation in        the line radius of curvature at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the buoyancy of said device,        -   including being utilized together with buoyancies of any            multitude of said devices    -    in order to increase the dynamic minimum in the variation in        the line radius of curvature at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   approximately neutral buoyancy of said device,        -   including being utilized together with approximately neutral            buoyancies of any multitude of said devices    -    in order to increase the dynamic minimum in the variation in        the line radius of curvature at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the submerged weight of said device,        -   including being utilized together with submerged weights of            any multitude of said devices    -    in order to increase the dynamic minimum in the variation in        the line radius of curvature at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the drag force on said device,        -   including being utilized together with drag forces on any            multitude of said devices    -    in order to increase the dynamic minimum in the variation in        the line radius of curvature at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the added mass of said device,        -   including being utilized together with added masses of any            multitude of said devices    -    in order to increase the dynamic minimum in the variation in        the line radius of curvature at any locality, including any        localities, along said line.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the mass of said device,        -   including being utilized together with masses of any            multitude of said devices    -    in order to increase the fatigue life of any component of said        line, including any multitude of lines, including any internal        component of said line cross section.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the buoyancy of said device,        -   including being utilized together with buoyancies of any            multitude of said devices    -    in order to increase the fatigue life of any component of said        line, including any multitude of lines, including any internal        component of said line cross section.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   approximately neutral buoyancy of said device,        -   including being utilized together with approximately neutral            buoyancies of any multitude of said devices    -    in order to increase the fatigue life of any component of said        line, including any multitude of lines, including any internal        component of said line cross section.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the submerged weight of said device,        -   including being utilized together with submerged weights of            any multitude of said devices    -    in order to increase the fatigue life of any component of said        line, including any multitude of lines, including any internal        component of said line cross section.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the drag force on said device,        -   including being utilized together with drag forces on any            multitude of said devices    -    in order to increase the fatigue life of any component of said        line, including any multitude of lines, including any internal        component of said line cross section.    -   Said relatively low average effective tension, including and        excluding said catenary line properties, being utilized together        with        -   the added mass of said device,        -   including being utilized together with added masses of any            multitude of said devices    -    in order to increase the fatigue life of any component of said        line, including any multitude of lines, including any internal        component of said line cross section.    -   Said reduction including        -   said reductions, in the dynamic component of said effective            tension,        -   including and excluding any reduction in said negative            component of said dynamic effective tension,    -    at any locality, including any localities, along said line,        including any multitude of lines, being achieved by said        arrangement favorably combining said relatively low average        effective tension, including and excluding said catenary line        properties, with any combination of said mass, said buoyancy,        said approximately neutral buoyancy, said submerged weight, said        drag force and said added mass of said device, including any        plurality of said devices of known design.    -   Said increase in said dynamic minimum in the variation of the        line radius of curvature at any locality, including any        localities, along said line, including any multitude of lines,        being achieved by said arrangement favorably combining said        relatively low average effective tension, including and        excluding said catenary line properties, with any combination of        said mass, said buoyancy, said approximately neutral buoyancy,        said submerged weight, said drag force and said added mass of        said device, including any plurality of said devices of known        design.    -   Said increase in the fatigue life of any component of said line,        including any multitude of lines, including any internal        component of said line cross section, at any locality, including        any localities, along said line being achieved by said        arrangement favorably combining said relatively low average        effective tension, including and excluding said catenary line        properties, with any combination of said mass, said buoyancy,        said approximately neutral buoyancy, said submerged weight, said        drag force and said added mass of said device, including any        plurality of said devices of known design.    -   Said use of novel devices according to this invention, which        feature modified technical characteristics involving any        combination of said mass, said buoyancy, said approximately        neutral buoyancy, said submerged weight, said drag force and        said added mass of said novel device anywhere along said line,        including any pluralities of lines used and any multitudes of        locations on said lines.    -   Said novel devices involving a use of arbitrary geometry shapes        designed to increase hydrodynamic drag and added mass of said        novel devices, in comparison with those of devices of known        design used on said lines.    -   Said geometric shapes including any three dimensional        arrangements of circles, ellipses, ovals, triangles, squares,        rectangles and other arbitrary polygons, arbitrary star figures,        helical strakes and any complex combinations of flat and three        dimensional shapes.    -   Any and all of said shapes having smooth edges and any and all        of said shapes having rugged edges.    -   Said shapes having        -   solid flat shape areas,        -   solid curved areas, which might incorporate a curvature,        -   and which might feature twisting,        -   and also said shapes featuring holes,        -   said shapes featuring slots,        -   said holes and said slots that might be used in order to            increase their drag and added mass enhancing effectiveness.    -   The areas and the aspect ratios of said devices that enhance the        drag and added mass in differing directions need not be the        same, in fact in a general case they would be different.    -   Said aspect ratio is defined herein as the square of its maximum        dimension presented to the flow divided by the surface area of a        given shape, presented to the flow along the mean normal vector        to the surface of the shape.    -   Said reduction, including said reductions,        -   in the dynamic component of said effective tension,        -   including and excluding any reduction in said negative            component of said dynamic effective tension,    -    at any locality, including any localities, along said line,        including any multitude of lines, by said arrangement favorably        combining said relatively low average effective tension,        including and excluding said catenary line properties, with a        use of said novel devices according to this invention that        feature modified technical characteristics involving any        combination of said mass, said buoyancy, said approximately        neutral buoyancy, said submerged weight, said drag force and        said added mass of said novel device.    -   Said increase in said dynamic minimum in the variation of the        line radius of curvature at any locality, including any        localities, along said line, including any multitude of lines,        being achieved by said arrangement favorably combining said        relatively low average effective tension, including and        excluding said catenary line properties, with a use of said        novel devices according to this invention that feature modified        technical characteristics involving any combination of said        mass, said buoyancy, said approximately neutral buoyancy, said        submerged weight, said drag force and said added mass of said        novel device.    -   Said increase in the fatigue life of any component of said line,        including any multitude of lines, including any internal        component of said line cross section, at any locality, including        any localities, along said line being achieved by said        arrangement favorably combining said relatively low average        effective tension, including and excluding said catenary line        properties, with a use of said novel devices that feature        modified technical characteristics involving any combination of        said mass, said buoyancy, said approximately neutral buoyancy,        said submerged weight, said drag force and said added mass of        said novel device.    -   Said reduction, including said reductions,        -   in the dynamic component of said effective tension,        -   including and excluding any reduction in said negative            component of said dynamic effective tension,    -    at any locality, including any localities, along said line,        including any multitude of lines, by said arrangement favorably        combining said relatively low average effective tension,        including and excluding said catenary line properties, with a        use of any combination of said known devices and said novel        devices.    -   Said increase in said dynamic minimum in the variation of the        line radius of curvature at any locality, including any        localities, along said line, including any multitude of lines by        said arrangement favorably combining said relatively low average        effective tension, including and excluding said catenary line        properties, with a use of any combination of said known devices        and said novel devices.    -   Said increase in the fatigue life of any component of said line,        including any multitude of lines, including any internal        component of said line cross section, at any locality, including        any localities, along said line, including any multitude of        lines, by said arrangement favorably combining said relatively        low average effective tension, including and excluding said        catenary line properties, with a use of any combination of said        known devices and said novel devices.    -   Dynamics decoupling, damping and added mass enhancing        arrangement including a single device and also including a        system of multiple devices and also including any plurality of        systems of such devices, which affect catenary line dynamic        motion in marine engineering; whereas said catenary line is        provided with said decoupling, damping and added mass enhancing        devices fitted on said catenary line fitted on said catenary        line along a segment of said line located in the vicinity of the        seabed.    -   Dynamic motion suppressing arrangement as claimed herein        utilizing said devices arranged on said catenary lines        essentially continuously, including arrangements in groups and        including distinctly located devices.    -   Dynamic motion suppressing arrangement claimed herein that is        used on any new built line of known configuration.    -   Dynamic motion suppressing arrangement claimed herein that        utilizes any decoupling, damping and added mass enhancing        device, including any plurality of such devices of known design.    -   Dynamic motion suppressing arrangement claimed herein, that        utilizes any decoupling, damping and added mass enhancing        device, including any plurality of such devices of novel design.    -   Line configuration involving any grouping of positively buoyant        devices, including continuously distributed said devices,        claimed herein, installed on said line so that most of said        grouping is installed in the lower ⅜ of the line suspended        length.    -   Line configuration involving any grouping of positively buoyant        devices, including continuously distributed said devices,        claimed herein, installed on said line so that most of grouping        is installed in the lower ¼ of the line suspended length.    -   Line configuration involving any multitude of positively buoyant        devices, including continuously distributed said devices,        claimed herein, installed on said line so that most of said        grouping is installed in the lower 3/16 of the line suspended        length.    -   Line configuration involving any grouping of approximately        neutrally buoyant devices, including continuously distributed        said devices, as described herein, installed on said line so        that most of grouping is installed in the lower ⅜ of the line        suspended length.    -   Line configuration involving any grouping of approximately        neutrally buoyant devices, including continuously distributed        said devices, as described herein, installed on said line so        that most of said grouping is installed in the lower ¼ of the        line suspended length.    -   Line configuration involving any grouping of approximately        neutrally buoyant devices, including continuously distributed        said devices, as described herein, installed on said line so        that most of said grouping is installed in the lower 3/16 of the        line suspended length.    -   Line configuration involving any grouping of negatively buoyant        continuously distributed devices, as described herein, installed        on said line so that most of said grouping is installed in the        lower ⅜ of the line suspended length.    -   Line configuration involving any grouping of negatively buoyant        continuously distributed devices, as described herein, installed        on said line so that most of said grouping is installed in the        lower ¼ of the line suspended length.    -   Line configuration involving any grouping of negatively buoyant        continuously distributed devices, as claimed herein, installed        on said line so that most of said grouping is installed in the        lower 3/16 of the line suspended length.

Line configuration involving any grouping of added mass and dragenhancing devices, including continuously distributed said devices, asdescribed herein, installed on said line so that most of said groupingis installed in the lower ⅜ of the line suspended length.

-   -   Line configuration involving any grouping of added mass and drag        enhancing devices, including continuously distributed said        devices, as described herein, installed on said line so that        most of said grouping is installed in the lower ¼ of the line        suspended length.    -   Line configuration involving any grouping of added mass and drag        enhancing devices, including continuously distributed said        devices, as described herein, installed on said line so that        most of said grouping is installed in the lower 3/16 of the line        suspended length.    -   Line configuration involving any multitude of said devices,        including continuously distributed said devices, as described        herein, installed on said line so that at least a part of said        distributed length with said devices installed stretches on both        side of the design touch down point in any design line        configuration.    -   Any multitude of added mass and drag enhancing devices, as        claimed herein, using arbitrary geometrical shapes according to        this invention intersect at wide range of angles including acute        angles and right angles.    -   Dynamic motion suppressing arrangements described herein that is        used anywhere on said line that involves a suppression of Vortex        Induced Vibrations.    -   Dynamic motion suppressing arrangement described herein that is        retrofitted to suppress motions on any existing, already        installed line.    -   The design optimization process described herein that is used in        the motion suppression optimization design.    -   Any field development and any field redevelopment project that        uses arrangements, devices and design processes described        herein.

This invention has been described with reference to example embodimentsthat present in detail the design arrangement invented and means toachieve the novel degree of the dynamic motion suppression of catenarylines used in marine engineering. Multiple variations and modificationsexist within the scope and spirit of the invention as described anddefined in the following claims.

1. Dynamics decoupling, damping and added mass enhancing arrangementincluding a single device and also including a system of multipledevices and also including any plurality of systems of such devices,which affect catenary line dynamic motion in marine engineering; whereassaid catenary line is provided with said decoupling, damping and addedmass enhancing devices fitted on said catenary line along a said linesegment located in the vicinity of the seabed, so that a segment of thesaid catenary line in the said vicinity of the seabed has any pluralityof segments, including a single segment, of a non-negative buoyancy,which includes any combination of approximately neutral and positivebuoyancies.
 2. Dynamic motion suppressing arrangement according to claim1 utilizing said devices arranged on said catenary lines essentiallycontinuously, including arrangements in groups and including distinctlylocated devices along said segment.
 3. Dynamic motion suppressingarrangement according to claim 1 that is used on any new built line ofknown configuration.
 4. Dynamic motion suppressing arrangement accordingto claim 1 that utilizes any decoupling, damping and added massenhancing device, including any plurality of such devices of knowndesign.
 5. Dynamic motion suppressing arrangement according to claim 1that utilizes any decoupling, damping and added mass enhancing device,including any plurality of such devices of novel design.
 6. Lineconfiguration involving any multitude of said devices, includingcontinuously distributed said devices, as described in claim 1 installedon said line so that most of said distributed length lies in the lower ⅜of the line suspended length.
 7. Line configuration involving anymultitude of positively buoyant devices, including continuouslydistributed said devices, as described in claim 1 installed on said lineso that at least a part of said distributed length with said devicesinstalled stretches on both side of the design touch down point in anydesign line configuration.
 8. Line configuration involving any multitudeof approximately neutrally buoyant devices, including continuouslydistributed said devices, as described in claim 1 installed on said lineso that most of distributed length lies in the lower ⅜ of the linesuspended length.
 9. Line configuration involving any multitude ofnegatively buoyant continuously distributed devices, as described inclaim 1 installed on said line so that most of said distributed lengthlies in the lower ⅜ of the line suspended length.
 10. Line configurationinvolving any multitude of added mass and drag enhancing devices,including continuously distributed said devices, as described in claim 1installed on said line so that most of said distributed length lies inthe lower ⅜ of the line suspended length.
 11. Any multitude of addedmass and drag enhancing devices as claimed in claim 1 using arbitrarygeometrical shapes according to this invention intersect at wide rangeof angles including acute angles and right angles.
 12. Dynamic motionsuppressing arrangement according to claim 1 that is retrofitted tosuppress motions on any existing, already installed line.
 13. The designoptimization process as described in claim 1 that is used in the motionsuppression optimization design.
 14. Any field development and any fieldredevelopment project that uses arrangements, devices and designprocesses described in claim
 1. 15. An apparatus for adjusting the localbuoyancy of a subsea line selected from the group consisting of risers,flow lines, control lines and umbilical lines said subsea line having afirst end attached to a device on the seafloor and a second endproximate the sea surface comprising: at least one buoyancy controlmodule located so as to configure said subsea line in a double-catenaryconfiguration said buoyancy control module located such that, in use, itis at a depth that is at least about 60 percent of the local waterdepth, said subsea line being free to move in response to movement ofthe second end thereof.
 16. An apparatus as recited in claim 15 whereinthe buoyancy control module is located, in use, at a depth that is lessthan about 90 percent of the water depth.
 17. An apparatus as recited inclaim 15 wherein the buoyancy control module is located, in use, at adepth that is at least about 80 percent of the local water depth.
 18. Amethod of stabilizing the touchdown point of subsea line selected fromthe group consisting of risers, flow lines, control lines and umbilicallines, said subsea line having a first end attached to a device on theseafloor and a second end proximate the surface of the sea comprising:providing at least one buoyancy module on the subsea line located so asto configure said subsea line in a double-catenary configuration whereinthe at least one buoyancy module is located such that, in use, it floatsat a depth which is greater than about 60 percent of the water depth.19. A method as recited in claim 18 wherein the buoyancy module islocated at a depth which is less than about 95 percent of the waterdepth.
 20. A method as recited in claim 18 wherein the buoyancy module,in use, floats at a depth which is greater than about 80 percent of thewater depth.
 21. A method as recited in claim 18 wherein the buoyancymodule, in use, floats at a depth which is greater than about 90 percentof the water depth.